The intrinsic oxide decomposition reaction Si + SiOz-^ 2SiO t at the Si02/Si interface is shown to be nucleated at existing defect sites prior to the growth of physical oxide voids. At lower temperatures than needed for void formation, these defects become electrically active, leading to low-field dielectric breakdown unless sufficient O2 is available (low concentrations). The systematics of the required O2 suggests strongly that it reverses the initial decomposition by reoxidizing the SiO product at the interface.
The influence of annealing in vacuum and in controlled low-pressure oxygen ambient on breakdown characteristics of thin (∼500 Å) SiO2 films on Si (100) has been studied under ultrahigh vacuum conditions for temperatures 750–900 °C and controlled O2 partial pressures in the range 10−6–5×10−2 Torr. Dark current-voltage measurements on Al-gate capacitors show that vacuum annealing causes low-field self-healing breakdown by the formation of local defects in the oxide. This degradation of breakdown characteristics is suppressed by the presence of sufficient O2 in the annealing ambient, such that the O2 partial pressure must exceed the SiO equilibrium partial pressure by a factor of ∼100×. This behavior suggests that low-field breakdown is a consequence of oxide decomposition (Si+SiO2→2SiO↑) at defects in the oxide, which is suppressed by reoxidation of the volatile SiO reaction product.
The formation of voids (or holelike defects) in 500-Å-thick thermal oxide films on Si(100) due to thermal decomposition of SiO2 during vacuum annealing at high temperatures (>1050 °C) has been studied as a function of temperature and time. The defect size distribution is sharply peaked and the density of the defects is essentially independent of annealing time. These observations suggest strongly that the void formation process is initiated at defect sites which are already present after oxidation. The kinetics of oxide void growth suggest the presence of a nucleation stage of the reaction prior to void growth.
Thermal SiO2 films on Si subjected to high temperature heat‐treatments in O2‐normalfree ambients are unstable because Si supplied from the substrate reacts with SiO2 to form volatile normalSiO . The SiO2 decomposition process occurs nonuniformly at the interface and results in the growth of voids in the oxide where the SiO2 is completely removed. The formation of these voids was investigated for poly‐Si oxides and oxides grown on crystalline Si substrates. The crystalline Si received special treatments before and after oxidation, which included preoxidation annealing, ion implantation of Ar or Si, Cu diffusion, and Ar sputtering of the surface. The results suggest that the SiO2 decomposition is initiated at crystalline defects in the substrate where the normalSiO production is locally enhanced. The growth of the voids is influenced by preoxidation annealing of the Si substrates. Metal impurities, especially Cu, do not increase the decomposition rate by a catalytic mechanism, but they may cause a secondary effect when precipitated at crystalline defects prior to oxidation.
The generation of hole traps in thermal SiO2 films on Si(100) has been characterized as a function of O2 partial pressure in the annealing ambient in order to address the chemistry associated with the hole traps. The annealing treatments were carried out in ultrahigh vacuum (base pressure∼5×10−9 Torr) without and with the presence of an intentional (controlled) partial pressure of O2 in the range 10−6–5×10−2 Torr. Hole trapping was characterized using the avalanche injection technique. Annealing in vacuum results in an increased hole trapping rate similar to that observed for high-temperature (T>900 °C) furnace annealing in N2. The hole trapping is reduced upon annealing in O2 containing ambients if the O2 partial pressure exceeds the SiO vapor pressure by at least one order of magnitude. Thus, the presence of sufficient O2 in the postoxidation annealing process suppresses hole trapping. These results appear analogous to the reduction in low-field breakdown when O2 is present, as recently reported. In both cases, it is likely that the O2 serves to reoxidize a defect related SiO product which is generated by Si-SiO2 reaction at the interface.
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